This invention relates to the development of oxidation resistant brazing filler metals for direct brazing of ceramics to ceramics or ceramics to metals to form strong joints that can withstand high service temperatures.
A key technology that will enhance or restrict the use of ceramic materials in high performance applications, such as advanced heat engines or heat exchangers, is the ability to reliably join simple-shape ceramic components to form complex assemblies or to join unit lengths of ceramic material to form large ceramic systems. Although ceramic joining technology has been highly developed over the past fifty years, very little has been done to develop brazing filler metals for joining ceramics for use at elevated temperatures, at high stress levels and in dirty environments.
Brazing of ceramics is considerably more difficult than brazing of metals, and the applicant knows of only one commercially available brazing alloy, sold under the trademark "Wesgo's Ticusil" (Ag-26.7-Cu-4.5-Ti wt. %) that will wet and bond to an oxide ceramic. However, the poor oxidation resistance of that alloy's constituents (Ag-Cu-Ti) and relatively low brazing temperature (950.degree. C.) prevent its use in many high temperature applications.
There are basically two brazing processes that can be used for joining ceramics. One is indirect brazing, in which the ceramic is coated with an active metal prior to brazing with a nonreactive commercial filler metal, and the other is direct brazing with filler metals specially formulated to wet and bond to both metals and ceramics.
Direct brazing of ceramics is considerably more difficult than brazing of metals as most brazing filler metals will not wet a ceramic surface. Although some filler metals have been developed that wet some ceramic materials directly without the need for precoating, there are several factors that prevent the use of these filler metals in the advanced energy conversion applications under development today. Without exception, these filler metals contain one or more of the reactive elements titanium, zirconium or hafnium that promote bonding to oxide-base ceramics by reducing the oxide of the ceramic and forming Ti- Zr- or Hf- oxides at the ceramic/filler metal interface. Unfortunately, this same strong oxide forming tendency also creates a problem because the filler metal is inherently susceptible to corrosive oxidation upon long term exposure to the atmosphere at high temperatures. Another problem with many of the compositions previously developed is that they contain the toxic element beryllium, an undesirable material in today's commercial applications. Also, the melting range (or solidus temperature) of some of these compositions is too low for service at 1000.degree.-1200.degree. C., the temperature range under consideration for advanced heat engine applications.
Direct brazing does avoid the development and application of what is, in many cases, the very sophisticated and expensive coating or metallizing treatment required for indirect brazing. Also, the inclusion of the active metal within the filler metal more effectively protects the active metal from oxidation during storage or while brazing than when the pure active metal is first used to coat the ceramic. Finally, the strength of the bond between a coating and ceramic substrate, and the corrosion resistance of the coating does not have to be of concern in direct brazing. Although direct brazing offers the above advantages when used for joining ceramics, there is still a need to develop filler metals with improved oxidation resistance at high temperatures to avoid the shortcomings of previous compositions while retaining these advantages.